1. Field of the Invention
The present invention relates generally to a motor driven type die tightening apparatus for an injection molding machine for injection-molding various kinds of articles of a synthetic resin wherein the apparatus is driven with the aid of an electric motor such as a servomotor or the like as a power source. More particularly, the present invention relates to improvement of a motor driven type die tightening apparatus of the foregoing type which assures that die opening, die closing and die tightening can be achieved at a high speed with a reduced quantity of electricity consumption.
2. Background Art
Various kinds of die tightening apparatus each operable with an electric motor as a power source for injection-molding various kinds of articles using a synthetic resin are already known in the art. Among them, three typical conventional die tightening apparatuses of the foregoing type are disclosed in official gazettes of Japanese Utility Model Publication Nos. 1-36587, 1-36588 and Japanese Patent Laid-Open Publication 61-193821.
According to one of the prior inventions, i.e., Japanese Utility Model Publication NO. 1-36587, a die tightening apparatus includes a vertically extending end housing, located opposite to a stationary die board having a stationary die half mounted thereon, to serve as a movable die board having a movable die half mounted thereon, and a plurality of die bars are horizontally bridged between the stationary die board and the end housing, i.e., the movable die board. One end part of the respective tie bars is fixedly secured to the stationary die board, while the other end part of the respective tie bars is inserted through the movable die board and designed in the form of a screw shaft adapted to be threadably engaged with nut members arranged in the movable die board. With this construction, rotation of the tie bars is transformed into linear movement of the movable die board via the threadable engagement of the screw shafts of the tie bars with the nut members of the movable die board. Thus, as an electric motor installed on the movable die board is rotationally driven, the tie bars are rotated by the electric motor via a rotational power transmitting mechanism, whereby die closing or die opening is achieved by the linear movement of the movable die board toward or away from the stationary die board. Incidentally, each die closing is effected by tightening a movable die half mounted on the movable die board against a stationary die half mounted on the stationary die board.
According to another one of the prior inventions, i.e., Japanese Utility Model Publication No. 1-36588, a plurality of tie bars each designed in the form of a screw shaft and horizontally bridged between a stationary die board and a movable die board are kept immovable, and an electric motor and a rotational power transmitting mechanism are arranged on the movable die board side. With this construction, as an electric motor is rotationally driven, the movable tie board is linearly displaced along the die bars via threadable engagement of the screw shafts of the tie bars with nut members arranged in the movable die board.
In addition, according to another one of the prior inventions, i.e., Japanese Patent Laid-Open Publication No. 1-193821, an electric motor and a rotational power transmitting mechanism are installed on a movable die board or a stationary die board.
With the conventional motor driven type die tightening apparatus of the type constructed such that the movable die board is displaced by rotating the respective tie bars, since it is necessary that one end of the long tie bars be rotated by the rotational power transmitting mechanism, a larger magnitude of torque, much more than if the other end of the tie bars on the nut member side is rotated by the foregoing mechanism, is required for displacing the movable die board. A problem of the conventional apparatus to be solved is that a high level of technique is required for the purpose of supporting the opposite ends of each long tie bar without any particular error.
In addition, with the conventional motor driven type die tightening apparatus of the type constructed such that the nut members are rotated for displacing the movable die board, since the electric motor, the nut members, the rotational power transmitting mechanism and associated components are arranged on the movable die board, the weight of the movable die board is unavoidably increased, resulting in a rotational load to be borne by the electric motor being increased correspondingly. Thus, a large-sized electric motor capable of outputting a large magnitude of torque is required. Additionally, since a large part of the weight of the movable board is borne by the screw shaft portions of the tie bars, the latter are liable to be readily deflected compared with ordinary tie bars. Thus, another problem of the conventional apparatus to be solved is that it is difficult to maintain a high accuracy of the parallel extension of the movable die board relative to the stationary die board.